Exhaust gas recirculation is a technique commonly used for controlling the generation of undesirable pollutant gases and particulate matter in the operation of internal combustion engines. This technique has proven particularly useful in internal combustion engines used in motor vehicles such as passenger cars, light duty trucks, and other on-road motor equipment. The exhaust gas recirculation technique primarily involves the recirculation of exhaust gas by-products into the intake air supply of the internal combustion engine. This exhaust gas thus reintroduced to the engine cylinder reduces the concentration of oxygen therein, which in turn lowers the maximum combustion temperature within the cylinder and slows the chemical reaction of the combustion process, decreasing the formation of nitrous oxide. Furthermore, the exhaust gases typically contain a portion of unburned hydrocarbon which is burned on its reintroduction into the engine cylinder, which further reduces the emission of exhaust gas by-products which would be emitted as undesirable pollutants from the internal combustion engine.
When utilizing EGR in a turbocharged diesel engine, the exhaust gas to be recirculated is preferably removed upstream of the exhaust gas driven turbine associated with the turbocharger. In many EGR applications, the exhaust gas is diverted directly from the exhaust manifold. Likewise, the recirculated exhaust gas is preferably re-introduced to the intake air stream downstream of the compressor and air-to-air aftercooler. For example, in many EGR applications the recirculated exhaust gas is reintroduced to the intake manifold.
Reintroducing the exhaust gas downstream of the compressor and air-to-air aftercooler is preferred due to the reliability and maintainability concerns that arise should the exhaust gas is passed through the compressor and aftercooler. However at some engine operating conditions, there is a pressure differential between the intake manifold and the exhaust manifold which essentially prevents many conventional EGR systems from being utilized. For example, at high speed, high load conditions in a turbocharged engine, the exhaust gas does not readily flow from the exhaust manifold to the intake manifold.
It is well known that lower intake manifold temperatures tends to reduce the formation of nitrous oxides found in the exhaust gases. Therefore cooling of the EGR is a technique that has been used to further augment the reduction of NOx where EGR is employed. Various EGR cooling subsystems have been disclosed in the related art including jacket water cooled EGR cooling systems and EGR systems diverting the recirculated exhaust gases through the engine intake intercooler or aftercooler.
Using a jacket water EGR cooler is the most common technique disclosed in the related art. However, one of the key challenges of cooling EGR by means of a jacket water cooler is the increase in heat rejection from the jacket water EGR cooler. The significant heat rejection to the jacket water cooling system due to the presence of a cooled EGR system requires sizable increases in the vehicle radiator. The other technique of utilizing the intercooler or aftercooler of the intake system presents problems from a fouling of various components on the intake system from the dirty exhaust gases recirculating therethrough.
Several other related art EGR cooling systems have been disclosed including U.S. Pat. No. 5,607,010 (Schonfeld). U.S. Pat. No. 5,607,010 (Schonfeld) discloses an EGR system that includes an EGR conduit or pipe and a pair of heat exchangers connected in series in the EGR pipe and each adapted to cool the exhaust gases to different predetermined temperature ranges wherein the exhaust gas is cooled to a temperature of about 250 degrees C.
What is needed, therefore, is a simple and inexpensive technique for recirculating exhaust gas from the exhaust manifold to the intake manifold at all engine operating conditions while efficiently cooling the recirculated exhaust gases to the desired temperature without fouling of the intake air system.